[Dell Precision M4800] Brightness can't be controlled on 14.04.1

[Po-Hsu Lin]

CID: 201305-13527 Dell Precision M4800

The screen brightness

The brightness control does not work at all on 14.04.1

Steps:
1. Install 14.04.1 + update
2. Test the brightness control with hotkeys
3. Test the brightness control by using the "Brightness and Lock" applet.

Expected results:
* Brightness control should work in step 2 and 3

Actual result:
* Brightness level would change with the hotkey, but the backlight level won't, nothing happened when you scroll the scrollbar in the "Brightness and Lock" applet.

Debugging information:
ubuntu@201305-13527:~$ sudo showkey -s
kb mode was ?UNKNOWN?
[ if you are trying this under X, it might not work
since the X server is also reading /dev/console ]

press any key (program terminates 10s after last keypress)...
0x9c
0xe0 0x54 0xe0 0xd4
0xe0 0x4c 0xe0 0xcc
0x1d
^Ccaught signal 2, cleaning up...
ubuntu@201305-13527:~$

ubuntu@201305-13527:~$ sudo showkey -k
kb mode was ?UNKNOWN?
[ if you are trying this under X, it might not work
since the X server is also reading /dev/console ]

press any key (program terminates 10s after last keypress)...
keycode 28 release
keycode 225 press
keycode 225 release
keycode 224 press
keycode 224 release
keycode 29 press
^Ccaught signal 2, cleaning up...
ubuntu@201305-13527:~$

ubuntu@201305-13527:~$ ls /sys/class/backlight/
acpi_video0

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: linux-image-3.13.0-44-generic 3.13.0-44.73
ProcVersionSignature: Ubuntu 3.13.0-44.73-generic 3.13.11-ckt12
Uname: Linux 3.13.0-44-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.6
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: ubuntu 1635 F.... pulseaudio
                      ubuntu 2242 F.... pulseaudio
 /dev/snd/controlC0: ubuntu 1635 F.... pulseaudio
                      ubuntu 2242 F.... pulseaudio
CRDA:
 country TW:
  (2402 - 2472 @ 40), (3, 27)
  (5270 - 5330 @ 40), (3, 17), DFS
  (5735 - 5815 @ 40), (3, 30)
CurrentDesktop: Unity
Date: Fri Jan 16 00:56:25 2015
HibernationDevice: RESUME=UUID=4e3c0183-4d3f-495e-8d6a-57e2b23e6412
InstallationDate: Installed on 2015-01-16 (0 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
MachineType: Dell Inc. Precision M4800
ProcFB: 0 nouveaufb
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-44-generic root=UUID=3edd6366-27b1-41af-88d4-ac1abe216d8f ro quiet splash vt.handoff=7
RelatedPackageVersions:
 linux-restricted-modules-3.13.0-44-generic N/A
 linux-backports-modules-3.13.0-44-generic N/A
 linux-firmware 1.127.11
RfKill:
 0: phy0: Wireless LAN
  Soft blocked: no
  Hard blocked: no
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/03/2014
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A11
dmi.board.name: 0FVDR2
dmi.board.vendor: Dell Inc.
dmi.board.version: X02
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA11:bd12/03/2014:svnDellInc.:pnPrecisionM4800:pvr01:rvnDellInc.:rn0FVDR2:rvrX02:cvnDellInc.:ct9:cvr:
dmi.product.name: Precision M4800
dmi.product.version: 01
dmi.sys.vendor: Dell Inc.

[Brad Figg (brad-figg)]

This change was made by a bot.

[Po-Hsu Lin (cypressyew)]

WORKAROUND:
Adding acpi_osi=\"!Windows 2012\" to grub could fix this issue.

[Po-Hsu Lin (cypressyew)]

Also, if the "Enable Switchable Graphics" option in BIOS is set to true, the brightness control would work.
This option will make two more backlight interface available.
ubuntu@201305-13527:~$ ls /sys/class/backlight/
acpi_video0 acpi_video1 intel_backlight

[Anthony Wong (anthonywong)]

This machine was enabled with Precise and Quantal, so the new kernel regressed.

[Po-Hsu Lin (cypressyew)]

With the proprietary driver nvidia-331 installed, the brightness control could work as expected.
(Switchable Graphic disabled.)

[Richard Laager (rlaager)]

I can confirm this on Vivid with the proprietary nvidia driver (version 346) installed. With switchable graphics disabled, I have an acpi_video0 backlight that does nothing. That is, the contents of /sys/class/backlight/acpi_video0/backlight changes, but the screen's brightness does not. If I enable switchable graphics, then I have only a /sys/class/backlight/intel_backlight, and it works.